Ultra-low temperature cofired ceramics based on Li ₂WO ₄ as perspective substrate materials for terahertz frequencies

With the development of low dielectric permittivity materials having an ultra-low sintering temperature, testing their dielectric properties at terahertz frequencies suitable for 6G communication systems and implementation of the fabricated materials in ultra-low temperature cofired ceramics (ULTCC) were the main goals of the research. Lithium tungstate Li₂WO₄ was synthesized by a solid-state reaction and used for the preparation of green tapes and test structures with cofired internal conductive layers, which are destined for substrates of microwave and submillimeter wave circuits. Sintering behavior, thermal effects, and mass changes of the green tapes during heating were studied using a hot-stage microscope, differential thermal analysis, and thermogravimetry. A single-phase composition was revealed for being undoped and doped with AlF₃–CaB₄O₇ ceramics. The impact of frequency, temperature, the addition of AlF₃–CaB₄O₇ and CuBi₂O₄ dopants, and sintering temperature was the subject of in-depth characterization of dielectric properties in a terahertz region. A glass-free composition, ultra-low sintering temperature of 590–630 ℃, low roughness of the green tapes, dense microstructure, compatibility with Ag conductors, low and stable dielectric permittivity of 5.0–5.8 in a broad range of 0.2–2 THz, and low dielectric loss of 0.008–0.01 at 1 THz are the main advantages of the developed ULTCC substrates.